Thirty children with childhood Attention Deficit Disorder (ADD) have been treated with a selective or nonselective MAO inhibitor. The major findings to date are that both clorgyline, a selective MAO-A inhibitor, and tranylcypromine, a nonselective inhibitor, were effective in decreasing hyperactivity and improving attention. There was a significant decrease in urinary MHPG excretion which showed some correlation with behavioral improvement both on MAOIs and on d-amphetamine. However, this decrease in MHPG persisted for several weeks after drugs were stopped while there was immediate behavioral rebound off drug for both amphetamine and for the MAOIs. The study supports alteration of catecholamine metabolism as mediating the effects of stimulant medication. A controlled trial of 1-deprenyl, a selective MAO-B inhibitor, is ongoing.